Reversing hot mills, also called Steckel mills, are used as hot strip mills for hot-rolling slabs of stainless steels, special steel, etc.
FIG. 1 shows schematically a reversing rolling mill 1 having a relatively high temperature where a cast slab of steel is passed through rolls 2a and 2b several times. The rolls 2a and 2b are in nip contact with each other and when the slab travels trough the nip N, the thickness of the slab is reduced to a strip 3 having a thinner thickness.
On opposite sides of the rolling mill 1 there are arranged two coiler furnaces 4. When processing the strip with the rolling mill 1, it is alternately wound up on and unwound from the opposed coiler furnaces 4, each comprising a rotating, cylindrical coiler drum 5. The purpose of the coiler furnaces is to maintain the temperature of the strip between passes through the nip N. The coiler furnaces are maintained at a temperature, for example, of about 900 to about 1050° C.
In operation, the strip 3 passing through the nip N is led to the nearer coiler furnace 4 and wound onto its respective coiler drum 5. Subsequently, the strip 3 is unwound from the coiler drum 5 as the strip is fed back through the nip N. The process passing the strip through the nip N and winding and unwinding it is repeated until a desired thickness of the strip is reached.
A coiler drum is a hollow cylinder having an outside diameter of at least about 1000 mm, a length of about 2000 to 5000 mm and a great wall thickness of about 30 to 150 mm. The coiler drum is usually prepared by casting of a heat resistant alloy. The working surface of a coiler drum, that is the surface that becomes next to the strip to be wound, is usually flat or grooved.
As can be realized from the description above, the coiler drum is working in a very harsh environment. The temperature in the coiler furnace is high. The surface of the coiler drum is also repeatedly subjected to high tightening force when winding the steel strip on the drum. This causes fatigue fractures or other cracks to the surface of the coiler drum. The coiler drums also blister. These influence the durability of the coiler drum. They also leave marks on the steel strip that is being manufactured, thus deteriorating the quality of the product. The surface of the coiler drum is also exposed to an iron oxide film formed on the surface of the steel strip when rolling up the steel strip. In spite of descaling, part of the iron oxide film transfers on and sticks to the coiler drum and subsequently causes defects to the strip. This deteriorates the quality of the product.
Nowadays the problems caused by the blisters in the coiler drum surface are solved by shutting down the furnace so that the blisters can be hand-ground to reduce the blisters. The frequent shut downs result in high operating costs for maintaining the rolls and reduction of product throughput capability.
The coiler drums also tend to sag at current operating temperatures, which prevents operating the furnace at even higher temperatures. The sagging causes the drums to become eccentric in their rotation. This causes uneven revolving of the drum, which has an influence on the operation and durability of the coiler furnace and causes defects on the strip. Sagging also causes deformations, i.e. narrowing the slot of the coiler drum. As a consequence, the coiler drum has to be replaced.
The above mentioned problems have been tried to overcome by using different cast steel compositions when casting the coiler drums. One example is disclosed in U.S. Pat. No. 6,033,626. The composition presented in the patent is only a variation of a standard steel composition. It is soft in high temperatures in which the coiler drums are being used and thus there are problems caused by the deformation of the coiler drum. Also it can not refrain the blistering of the surface of the coiler drum.
US published patent application 2001/0013383 discloses a trinickel aluminide-based heat resistant alloy for a material for hearth rolls for heating furnaces. The material has a high creep rupture strength in temperature ranges over 1050° C. and excellent weldability. The problem is that trinickel aluminide-based materials are expensive. The material is suitable for casting rolls with a simple structure, i.e. a hollow cylinder, but it can not be used to cast objects with complex geometrical structures, such as a coiler drum. Moreover, when preparing the molten trinickel aluminide alloy, the reaction is strongly exothermical. Thus it is not possible to cast objects that are as big in size as coiler drums are.
Both of these patent publications disclose casting the entire drum or roll from the same material and as one piece.